Recently, an MRAM (Magnetic Random Access Memory) has been proposed as one type of magnetic memory device that functions as a memory device. The MRAM stores information by utilizing the reversal of the direction of magnetization in a magnetoresistive element such as a giant magnetoresistive (GMR) or a tunnel magnetoresistive (TMR) element.
In a magnetoresistive element, for example, a TMR element, used in the MRAM, a free layer composed of a ferromagnetic material, a nonmagnetic layer composed of an insulating material, a fixed layer composed of a ferromagnetic material, and an antiferromagnetic layer for directly or indirectly fixing the direction of magnetization of the fixed layer are stacked in order, and the resistance of a tunnel current varies depending on the direction of magnetization of the free layer. Thus, the MRAM can store information according to the direction of magnetization of the free layer in the magnetoresistive element; for example, it stores “1” when the magnetization is oriented in a certain direction, and “0” when the magnetization is oriented in another direction.
In order to write information in the magnetoresistive element, the MRAM also includes an electrode layer composed of a nonmagnetic conductor that is disposed at least adjacent to the free layer in the magnetoresistive element. A magnetic field higher than a magnetic field Hc necessary to reverse the direction of magnetization of the free layer is applied to the magnetoresistive element by a magnetic field generated by an electric current passing through the electrode layer so as to change the direction of magnetization, thereby writing information in the magnetoresistive element.
In such an MRAM, the size (planar area) of the magnetoresistive element tends to decrease, thus increasing the packaging density. Therefore, of course, the size of the free layer that reverses the direction of magnetization (switching) tends to decrease.
However, since the distance between both ends of the free layer, that is, the distance between the magnetic poles in the free layer, decreases with the size reduction of the free layer, a demagnetizing field generated in the free layer increases. The demagnetizing field reduces a magnetic field externally applied to the free layer. For this reason, the demagnetizing field has a large influence on the coercive force in the free layer. When the demagnetizing field increases, a stronger magnetic field must be applied in order that the free layer can perform switching. That is, when the demagnetizing field increases, the amount of current to be applied to the electrode layer to generate a magnetic field in the free layer needs to be increased. As a result, power consumption during information writing increases.
In order to inhibit the coercive force from being increased by such a demagnetizing field, for example, the dependency of the demagnetizing field on the element size may be reduced by decreasing the moment of the free layer (the product of the saturation magnetization Ms of the ferromagnetic material that forms the free layer, and the thickness t of the free layer). This is because the demagnetizing field Hd, the moment Ms×t, and the size W in the direction in which a magnetic field is applied to the magnetoresistive element (normally, the direction of easy axis of magnetization) have a relationship Hd=A×Ms×t/W (A is a proportionality constant). However, the ferromagnetic material that forms the free layer cannot be easily changed because it has a large influence on the MR ratio. For this reason, the free layer needs to be decreased in thickness in order to reduce the moment thereof. However, when the thickness of the free layer is too small (for example, several nanometers), problems may occur: for example, the free layer does not form a continuous film, and the thermal stability decreases. That is, since the thickness reduction of the free layer is limited, it cannot be necessarily said that the increase in coercive force due to the demagnetizing field can be inhibited by the thickness reduction.
Accordingly, an object of the present invention is to provide a magnetic memory device which can inhibit the coercive force of a free layer from being increased by a demagnetizing field, regardless of the thickness, moment, and the like of the free layer so that information can be written with a low power consumption even when the size of the magnetoresistive element is reduced.